PROCEEDINGS HAGI-IAGI Joint Convention Medan 2013 28 – 31 October Landslide Vulnerability on the Main Road of Malino – Sinjai Based on Geological Aspects A. M. Imran1, Busthan Azikin1,2, Sultan1,2 ) Lecturer of Geology Dept., Hasanuddin University, Kampus Unhas Tamalanrea, Km 10, Makassar 90245 Email: [email protected], Telf. 0411586227, fax.: 0411-586015 2) Doctoral Students of Civil Engineering Study Program, Hasanuddin University. Abstract The study area lies on the north-east slope of Lompobattang Mountian. Due to unstabil landslide often occurs both in small and large scale, especially in the rainy season. Morphology consists of steep slope mountaneous area with lithology of young volcanics. It was applied detail geological surface mapping that is involving lithology, morphology and geological structure. Measuring the resistivity of rocks was done by applying geoelectric resistivity at two locations. The wire was 500 meter widespread crossing Manipi – Malino road that parallel to the slope The result of the study relies that morphology of the study area is very steep with slope 60O. The are consists of intercalation between volcanics breccia, tuff and lava. The rocks are commoly highly altered and found crack/faults. Based on the analysis, it is concluded that the are three types of sliding plane: sliding plane lies between soil and tuff, discontinuities of crack and/or fault, and bedding plane between lava and overlying rocks. Key word: Vunerabilty index, volcanic breccias, sliding plane, geological control Sari Wilayah studi terletak pada lereng bagian atas di sebelah utara timur laut puncak G. Lompobattang. Wilayah ini sering mengalami longsoran baik skala kecil maupun skala besar pada setiap musim hujan akibat wilayah tersebut relatif kurang stabil. Morfologi wilayah ini merupakan daerah berbukit terjal dengan batuan penyusunnya adalah vulkanik muda. Metode yang dilakukan dalam penelitian ini adalah pemetaan geologi detail secara permukaan yang menyangkut litologi, morfologi dan struktur geologi. Pengukuran resistivitas batuan dengan alat geolistrik pada dua titik dengan jarak bentangan 500 meter. Pengukuran dilakukan dengan memotong jalan poros Malino – Manipi atau mengikuti kemiringan lereng. Hasil penelitin mengungkapkan bahwa morfologi wilayah studi sangat terjal dengan kelerengan mencapai 60O. Litologi penyusun wilayah penelitian adalah breksi vulkanik, tufa dan lava dengan posisi stratigrafi berselingan. Kondisi batuan telah terlapukkan kuat. Selain itu kondisi wilayah juga banyak ditemukan rekahan dan liniement serta gawir-gawir. Berdasarkan analisis tersebut maka disimpulkan bahwa terdapat bidang-bidang gelincir antara lapukan tufa dengan batuan asalnya, diskontinuitas oleh rekahan/patahan dan adanya lapisan lava (impermeabel) diantara lapisan tufa dan breksi vulkanik. Kata Kunci: Kerentanan longsor, batuan vulkanik, bidang gelincir, kontrol geologi Introduction Based on the data from Badan Nasional Penanggulangan Bencana (BNPB), there are 11 regencies in South Sulawesi which are vulnerable with natural hazard (especially landslide) namely Tanah Toraja, Luwu, Pinrang, Sidrap, Wajo, Soppeng, Bone, Sinjai, Bantaeng, Gowa dan Makassar (Harian Fajar, 5 Nopember 2010). The occurence of landslide also took place in many main roads in South Sulawesi. The quick survey of Busthan et al. (2011) found that the land slide occurred in many palces alaong the main road connecting of Malino (Gowa Regency) - Manipi (Sinjai Regency). This is due to the geological condition consisting the area. The lithology is mainly made up of young Lompobattang Volcanics which is not well compacted and highly weathering. The landslide occured in 2006 triggered flash flooding in Sinjai Regency, in which material of landslide filled the upstream of Mangottong River in east side G. Bawakaraeng and block the water flow of the river for a while. This flash flood have coused many losses in Sinjai Regency. At least 7 small landslides are identified along the main road of Malino – Sinjai that have been occurred in 2006. Study area is located on the northeastern slope of G. Lompobattang, or laying on the main road from Gowa Regency to Sinjai Regency. This study aims to analyze the vulnerability of landslides on the main road of Malino (Gowa Regency) – Sinjai Regency. PROCEEDINGS HAGI-IAGI Joint Convention Medan 2013 28 – 31 October Peak of G. Lompobattang enelitian Bone Bay Study Area structure is relatively parallel to the Walanae Fault which is the main fault in South Sulawesi. In addition to minor faults on the field are found many openings or fractures, with wide openings up to 20 cm, and the escarpment. Geoelectric resistivity observations at 2 stations indicates two potential locations of landslide (Figure 2). a) The potential location that has slip plane between the weathered tuff (resistivity values from 15 to 29.5 ohm.m) with basaltic intrusive rock (resistivity 400 - ohm.m 1000). This slip plane is a crushed zone fault that occur up to 25 meters depth. b) The second potential landslide locates at the upper part of the road. The slip plane lies between weathered tuff (resistivity 15 to 29.5 ohm.m) and its one (resistivity 50200 ohm.m) in the depth of 7.5 meters below surface. Figure 1 Study area and Topography of G. Bawakaraeng. Research Methodology Jalan Poros Manlino-Sinjai This research was carried out by several methods such as detail geological mapping in areas that potential for landslides and in areas that have been occured landslide including distribution of landslides material, type of material and material origin. Furthermore it was applied a geoelectric resistivity multichannel at two locations. The aim of applying this method is to determine the water content of subsurface which is considered as a slip plane. Result and Discussion Topographic conditions of the study area has steep slope upto 85O (Figure 1). Such steep slope is one of the factors that determine the stability of the slope. If there are enough triggers such as vibration on the road, or the heavy rain, it will tend to be unstable and eventually will occure landslide. Several researches concerning a slope control to the landslide on the volcanic region are namely by Kawamura, et al., (2009). Lithology also play an important role in slope stability. Geology of the study area is composed of volcanic belonging to the G Lompobattang. The volcanics consists of Miocene Camba Volcanics (Tmcv), Pliocene Baturappe-Cindako Volcanics (Tpbv) and Pleistocene Lompobattang Volcanics (Qlv) (Sukamto & Surpriatna, 1982). Measuremen location of the study is in West Sinjai District is dominanted by volcanic rocks of Lompobattang Volcanics seires. The rocks consists of volcanic breccia, tuff, agglomerate and basalt porphyry. According to Yowono (1989) G.Lompobattang is the youngest volcano in South Sulawesi and has cone types (strato volcano). This volcanic type is made up of intercalation of pyroclastic and lava rock. Geolocical structure developing in this area are faulting (liniamen) generally trending north - south. This Figure 2 Result of the geoelectric survey (Res2div) with cable line across the main road. Interpretation og this figure in figure 3. At the seconds station is located at the steep slope. Interpretation of geoelectric survey shows the potential landslide lies in the upperpart of the main road within the weathered tuff (soil). The slip plane lies between the weathered tuff (resistivity 2,6 – 25 ohm.m) and its fresh tuff (resistivity 35 – 200 ohm.m) with a depth until 10 meter (Figure 3). Several researches that used geoelectric survey such as Sule et al. (2007) in Panawangan, Ciamis. Based on the above data it can be explain that the main factors of landslide are slope and lithology. The lithology made up the study area is dominated by the young volcanics of Lompobattang Volcanics. The rocks are easely weathered. The field survey shows that many small size of rock fall found in the study area. Furthermore the high weathered rocks also have a high swelling factor (Nurjamil, et al.. (2005). PROCEEDINGS HAGI-IAGI Joint Convention Medan 2013 28 – 31 October Rainfall in the study area were high with an average of 200 mm / month. Thus rainfall is very supportive as a trigger landslides. In addition, there are many springs found in the study area. Bidang Gelincir Figure 3 Interpretation of figure 2 indicating 2 slip plane of potential landslide. Based on the geoelectric survey done in 2 locations and detailed geological mapping, it can be interpreted that the landslides that have occurred in the study area is debris slump type (Varnes, 1978) or rotational landslide (USGS. 2003). Cross section presented in figure 2 and 3 has a semicircular slip plane. Based on the mechanism of landslides that have occurred in that area supported by field data, landslide occuring in the study area is slum or rotational type (Figure 5). In addition to the lithologic condition, stratigraphy also play an important role for landslide susceptibility. Study of Yuwono (1989) found that the lithology of Lompobattang Volcanics is intercalting between tuff and lava. Lava is an impermeable layer, while tuff is highly weathered. By the presence of lava within the tuff dan volcanic breccia it can be acs as a slip plane due to its impermeable.characteristics. The second main factor that plays an important role is the geological structure. The structure found in the study area such as faults, fractures and bedding plane. Fracture openings generally have sizes from cm to 20 cm. Such opening is potential to occur landslide (Figure 4). The geological structure also contributes to the occurrence of mass movement. Sumaryono and Triyana (2011) explained that the distribution of fault around the peak of G Bawakaraeng is commonly normal faults trending relatively north – south. Jalan Raya Bidang Gelincir Massa tanah potensi bergerak W cos = R W sin = T g Massa tanah stabil Keterangan: - R= Gaya Penahan (W cos α - T = Gaya penggerak (W sin α - α = sudut kemiringan bidang geincir Figure 5 Slope condition along the main road connecting Malino and Sinjai (above), and simplified of landslide susceptibility in the study area. Conclusion Figure 4. Crack of the weathered tuff as a sign to occur landslide. Landslide susceptibility in the main road of Malino - Sinjai is controlled by geological factors such as slope, lithology and geological structure. These factors are steep slopes with slope between 35 - 85O, rock type is young and highly weathered volcanic rocks, as well as it is found a number of PROCEEDINGS HAGI-IAGI Joint Convention Medan 2013 28 – 31 October discontinuous such as fractures / faults and escarpmentescarpment. There are three types of slip plane namely at the boundary between weathered tuff and its fresh one, at discontinuity especially faulting and the presence of lava/rock laharik. While the type of mass movement is debris avalanches and/or rotational slump. REFERENCES Badan Nasional Penanggulangan Bencana, 2010, Rencana Nasional Penanggulangan Bencana Tahun 20102014, Peraturan Kepala BNPB No.3 Tahun 2010, Harian Fajar, 5 Nopember, 2010 Kawamura, S., Miura, S., Ishikawa, T., & Ino H. (2009). 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